Omio (formerly GoEuro) helps you compare bus, train and flight connections and cooperates with Deutsche Bahn, Flixbus and other transport providers. You can buy tickets via Omio as you would on Bahn.de, for example. Currently, there’s a voucher code that takes 10% off bookings:

If you are invited to Omio or invite a friend and they make a booking, you will be credited €10 to your account which you can redeem on a booking of at least €40:
If you don’t know anyone that uses Omio, you can use our referral link. Register using this link or add the following code to your existing account: peerl2f5d8p2d
Unfortunately, Flixbus bus tickets cannot be purchased with the coupon. You will be forwarded directly from Omio to Flixbus where the coupon is not valid.
Redeem Coupon
You can redeem a coupon code at the end of the booking process. Next to Use a discount you have to click on the arrow and the coupon field will open:

After clicking on Apply, the value of the discount will immediately be subtracted from the total if the coupon code is valid.